OF PALEOZOIC STELLEKOIDEA. 141 



genera and show the line of development from the deeply stellate 

 primary form to the pentagonal genera with well-developed inter- 

 brachial arcs. 



PETRASTER RIGIDUS (Billings). 

 Plate 27, fig. 5. 



Palasterina rigidus BILLINGS, Geol. Surv. Canada, Rep. of Progress, 1853-1856, 



1857, p. 291. 

 Petraster rigidus BILLINGS, Geol. Surv. Canada, Can. Org. Hem., dec. 3, 1858, 



p. 80, pi. 10, fig. 3a (not fig. 3bHudsonaster matutinus} . WRIGHT, Mon. 



British Foss. Echinod., Oolitic, vol. 2, pt. 1 (Palaeontogr. Soc. for 1861), 



1862, p. 29. HALL, Twentieth Rep. N. Y. State Cab. Nat. Hist., 1868, p. 



294; rev. ed., 1868=1870," p. 337. 



Description of 1858. "This species has much the aspect of an 

 Astropecten; the disk is one-fourth the whole diameter, the rays 

 rather slender and uniformly tapering, the angles between the bases 

 of the rays rounded. The plates [of the actlnal side] which appear to 

 be adambulacral [increase very little in size from the tips of the rays 

 toward the mouth], are quadrate and a little convex; [the adambula- 

 cral columns terminated in the mouth area by a pair of pointed 

 oral plates and not by a single plate as shown in the original illustra- 

 tion], the marginal [inframarginal] plates oblong, and also convex 

 [certainly not less than 16 and probably 20 in each column, increasing 

 rapidly in size toward the axils, whsre there is a single large axillary 

 plate] ; the disk plates [accessory interbrachials] consist of three at 

 each angle [one orally and two distally], and a single row [of not more 

 than seven plates] on each side of the ray, but extending only one- 

 third or one-half of the length of the ray; they all lie between the 

 [infra] marginal and adambulacral plates. [Abactinal side unknown.] 

 The specimen figured was about 2 inches [or 50 mm.-] in diameter 

 when perfect ; width of disk half an inch, and of rays at the base about 

 three lines." 



Formation and locality. Trenton limestone, Ottawa, Canada. 

 Holotype No. 140 la is in the Victoria Memorial Museum, Ottawa. 

 The species has also been identified by Springer in the Lower Trenton 

 (Kirkfield) at Kirkfield, Ontario. 



Remarks. Hall and Billings discussed their asterid genera and 

 species at different times and finally the former examined Billings' s 

 material. In this connection Hall showed that figure 3& of Petraster 

 rigidus was based on the actinal side of Hudsonaster matutinus. 

 Regarding figure 3a, which is the holotype of this species, he in 1870 

 wrote as follows: 



"The specimen illustrated in figure 3a has a few small intercalated 

 plates between the marginal and ambulacral [adambulacral] ranges 

 in two of the axils of the rays, and there are a smaller number of gran- 

 ules in a similar position but unequally distributed on one side of 



